Metal halide lamps have two basic configurations; those with an outer envelope and those without. In the former, the lamp’s basic construction (see Figure 1) is an inner envelope (called the arc tube), which contains the arc, and an outer envelope (called the bulb) which filters out ultraviolet radiation (UVR) and shields the inner arc tube. These lamps are typically single-ended (SE) and use a threaded mount to screw into a socket. The second lamp configuration lacks the outer envelope and typically has two ends (double-ended, DE) that need to be inserted into a socket, as we shall discuss shortly.
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Double-ended lamps, on the other hand, are designed to be mounted in a pair of sockets, one at each end. Also, DE lamps do not have the large outer envelope typically found on single-ended lamps that is used to limit the lamp’s UV radiation. Double-ended lamps require an additional safety glass to be installed in the lamp’s fixture in order for them to be safe for your tank’s inhabitants.